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THE USE OF 



SILICA GRAVEL IN CONCRETE 



BY 



E. J. DE SMEDT 



ANALYTICAL AND CONSULTING CHEMIST 



NEW YORK 



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THE USE OF 



Silica Gravel in Concrete. 



BY 



E. J. DE SMEDT, 



ANALYTICAL AND CONSULTING CHEMIST, 



NEW YORK. 



/m. 

NEW YORK : 

MARTIN B. BROWN, PRINTER, 

49 and 51 Park Place. 



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THE USE OF 

SILICA GRAVEL II CONCRETE 



The use of hydraulic concrete for all kinds of construction, 
sea-walls, and pavements lias become so general that the quality 
of the materials of which it is made is an item of great interest 
to engineers, contractors and tax-pavers. Engineers in making 
specifications in most instances call for broken stone, to the ex- 
clusion of other materials— gravel, etc., and I find that many 
persons who have not given the question proper consideration, 
entertain the theory that concrete made of broken stone alone is a 
superior mixture and should only be used. 

Whenever my opinion has been asked as to the advisability 
of allowing gravel or pebbles to be substituted in whole or in part, 
I have invariably answered that if they were clean, hard and of 
proper size, they would make equally as good, if not better, con- 
crete than broken stone. 

To procure good results in concrete it is essential that the 
sand, broken stone or gravel should be of a silicious composition, 
clean, hard, free of clay, organic matter or other impurities. 
As much care should be taken in the selection of sand and gravel 
as in that of cement. 

The theory that the irregular faces and sharp edges of 
broken stone contribute to the adhesion of the mortar with which 
they are mixed is incorrect, and this has been demonstrated by 
practical experience. 

It is a well-known fact that hydraulic cement shows greater 
adhesion to clean silica gravel than to broken limestone, and there- 
fore produces better results in concrete. 

Silica and silicates with hydraulic limes or cements make con- 
crete or artificial stone of a superior quality, on account of their 



mutual chemical affinity. The sea gravel generally used in the 
vicinity of New York in making concrete, beton, etc., has this 
advantage, that while it can be had in uniform sizes it is irregular 
in shape and packs solid. 

Proofs of the Foregoing. 

Gravel concrete has been used with great success in the con- 
struction of some of the largest works in New York and vicinity, 
principally in the foundation of the Race Rock Lighthouse, the 
largest single mass of concrete known to be placed under light- 
house construction. Gravel concrete was also used exclusively in 
the foundations of both towers of the New York and Brooklyn 
Bridge, about 8,000 tons of concrete filling being used in the 
New York caisson alone. The load carried by each square foot 
of this foundation was over seven tons. It was completed in 
1872, and there has been no appreciable settlement since. This 
concrete was made of one part Rosendale cement, two parts sand 
and four of beach gravel, all by measurement. The eminent 
engineer, C. C. Martin, C. E., was the chief engineer and super- 
intendent of the New York and Brooklyn Bridge, this con- 
crete having been made and put in place under his supervision. 

Gravel concrete was also used some twenty years ago in the 
reservoir in Prospect Park, built by the same engineer, Mr. Mar- 
tin, and holding twelve feet of water. It was made, bottom 
and sides, of gravel concrete with Rosendale cement, and is in 
perfect condition to-day. 

Race Rock Light. 

In answer to an inquiry as to his experience in the use of 
gravel for concrete, the well-known engineer and contractor, F. 
Hopkinson Smith, Ass. M. Soc. C. E., wrote under date of 
December 15, 1892, as follows : 

"In 1871 I was awarded the contract by the United States 
Lighthouse Establishment for the construction of the Race Rock 



Lighthouse, situated in Long Island Sound, two miles from the 
westerly end of Fisher's Island. The problems to be solved were 
the construction of a foundation capable not only of bearing the 
weight of the superstructure and resisting ordinary wave action. 
but also of withstanding the thrust caused by the immense fields 
of floating ice flowing out of the Connecticut river, with a 
normal velocity of six miles an hour — the only obstacle in the 
way being this lighthouse. The force of this pressure may be 
estimated by imagining a situation with the thermometer at zero, 
the wind forty miles an hour blowing with the tide and the ice 
twelve inches thick. As matter of fact, this has occurred several 
times since the light has been built, the ice piling upon the 
structure in a mass thirty feet high. 

To meet these problems successfully, an artificial island was 
constructed in the form of an ellipse, one hundred and seventy-live 
feet by one hundred and twenty -Ave. The interior of this island 
-was then excavated, leaving a circle with a diameter of about one 
hundred feet, in which was placed four iron bands, of an aggre- 
gate height of twelve feet, filled with concrete mixed as follows : 
One part English Portland cement, two parts sand, three parts 
screened gravel and three parts broken stone (trap), making, by 
measure, one cubic yard of mixed concrete to the barrel of 
cement of four hundred pounds. The mass, when placed, was 
twelve feet thick, sixty-nine feet in diameter on the bottom and 
fifty-seven feet on top, and contained twenty-nine thousand 
cubic feet of concrete. Upon this was built a masonry pier in the 
form of a frustum of a cone, faced and backed solid with Cape 
Ann granite, fifty-four feet in diameter and twenty-eight feet 
high, upon which was placed a two-story dwelling of granite, the 
whole superstructure weighing about live thousand tons. 

All of the concrete in this mass was mixed on the site in a hori- 
zontal concrete mixer, driven by a small portable engine, the prod- 
uct being then run into iron valve buckets, holding one cubic 
yard each, which were then lowered under and through the water 
and dumped so as to form a continuous mass. Only salt water 



6 

was used. "When the last and top band was filled (it came just to 
mean low-water mark), the whole surface was leveled off and 
cement mortar floated so as to make it level. 

Recent inspection of this work, made under the special super- 
vision of my superintendent, Captain Thomas A. Scott, who 
personally conducted much of the diving operations at the site, 
shows that this mass of concrete is absolutely intact ; not the 
slightest abrasion occurs even around the outside edge, while 
no part of the masonry shows the slightest deterioration, the 
mortar joints still remaining unbroken, showing that no settling 
had been possible. 

In answer to your inquiry as to my own experience and 
practice in mixing large masses of concrete, I have to say that 
I have always advised the use of gravel, to be used in equal 
parts with broken stone. 

First — Because its rounded surfaces pack more closely in the 
interstices of the broken stone, and 

Second — Because there is no possibility of ' arching,' which 
sometimes occurs with the exclusive use of broken stone, even 
when the presumption is that the concrete has been properly 
rammed. 

Within two years past the U. S. Lighthouse Establishment 
accepted my suggestion to incorporate gravel with the broken 
stone in the foundation of a lighthouse constructed by me for 
them in Cold Spring Harbor, Long Island Sound, as being pref- 
erable, although the specifications indicated all broken stone. 
This lighthouse, however, was much smaller in dimensions than 
the Race Rock structure, requiring only about five hundred yards 
cf concrete to fill the iron caisson. 

Very respectfully, your obedient servant, 

F. IIopkinson SjtflTII, 

Ass. M. Soc. C. E." 



Mr. Smith's views as to the value of pebbles in making con- 
crete are in conformity with the practice of the Lighthouse 
Department, as shown in the following specifications of a sea-wall 
constructed by his firm under a contract with the Lighthouse 

Board. 

11 Specifications for sea-wall concrete at Staten Island for the 
Lighthouse Board, contract dated June, 1SSS. 

" One part Portland cement, two parts sharp sand, three parts 
pebbles, four parts broken stone, not larger than will pass through 
a two- inch ring." 

And in the extension of the same wall as is shown by the 
following extract from specifications for foundation for sea- 
wall at Staten Island, by Major D. P. Heap, U. S. Eng., 
November 28, 1S92: 

Excavation and Concrete Filling. 

"After the coffer dam has been completed, the mud and soil must 
be pumped out to hard bottom and then be covered with two layers 
of concrete in bags (the bags to be sewed so as to leave no fag ends), 
and the coffer dam then filled with concrete to five and one-half feet 
below mean low water, and the last layer of concrete allowed to set for 
twenty-four hours. The concrete must be composed of one part Port- 
land cement of acceptable quality, two parts clean, sharp sand, three 
parts of pebbles and four parts broken stone not larger than will pass 
through a two-inch ring." 

Forty-second Street Reservoir. 

The following facts in reference to the old Croton Aqueduct, 
and the Reservoir at Forty-second street and Fifth avenue, fur- 
nished by Mr. James Symington, are of interest in this connec- 
tion, and are further evidence of the value of pebbles in the mak- 
ing of concrete : 

" Siu — The original specifications of 1S38, for the construction 
of the Croton Aqueduct, gate-houses and distributing reservoirs 
are signed by John B. Jervis, Chief Engineer, and are now filed 
in the office of the Chief Engineer of the Department of Public 
Works, Kew York City. 



They require for concrete, ' that hydraulic lime and sand 
shall be mixed with water to the consistency of thin mortar and 
into this shall be incorporated broken stone three-quarters to an 
inch and a half in diameter or gravel and pebbles not exceeding 
one inch diameter, in such proportions as shall be determined by 
the Engineer.' 

The custom of the Engineer of the Croton Aqueduct has 
been to use for concrete either broken stone or gravel, as might 
be most convenient at the location where the work was being 
done, and such is the rule at the present time, gravel concrete being 
considered equal to that made from broken stone. There seems to 
be no official record of the proportions, but the present distin- 
guished Chief Engineer, George W. Bird sail, C. E., states that 
the custom of his office has been to use one part cement, two 
parts sand and three parts gravel or stone, making a concrete 
which is practically water-tight. In the construction of the dis- 
tributing reservoir at Eorty-second street, Kew York City, gravel 
was principally used, because it could be delivered cheaply at that 
site from the north shore of Long Island. The concrete in the 
old Croton Aqueduct is also largely composed of gravel or pebbles, 
and its solidity is not exceeded by any work in the world, it 
having stood the tests of pressure and time. The part of the old 
Aqueduct south of the Harlem river, when removed, showed the 
pebble concrete to equal solid rock in density and hardness, and 
the part still standing near One Hundred and Seventh street bears 
mute witness to the knowledge and fidelity of the engineers under 
whose direction it was constructed. 

The specification for the new Croton Dam now under con- 
tract requires that ' the concrete shall be formed of sound 
broken stone or gravel not exceeding two inches at their greatest 
diameter. All stone in any way larger is to be thrown out. The 
materials to be cleaned from dirt and dust before being used ; to 
be mixed in the proportion of four parts stone to one part cement.' 
The proportion of sand in the mortar is left to the discretion of 
the engineer. 



The eminent veteran architect, Mr. James Renwick. under 
whose design the Distributing Reservoir at Forty-second street 
was constructed, and who is to-day the only survivor of the band 
under whose direction this great public work was carried out, 
has kindly consented to make public a statement as follows: 

' The composition and materials used in the construction of the 
foundation of the Distributing Reservoir of the Croton Aqueduct, 
which was built under my supervision, I beg leave to state was 
composed of one part of Rosendale cement, three parts of clean, 
sharp sand, and four parts of gravel from the north shore of 
Long Island, which was nearly pure white quartz of size not 
exceeding one inch in diameter.' (Signed) James Renwick. 

The Distributing Reservoir has twelve inches of concrete over 
the whole bottom covering the puddle, and holds twenty million 
gallons of water with a depth of thirty-six feet. 

James Symington. 
X. Y., February 25, 1893." 



This is additional evidence that clean hard silica gravel or 
pebbles make concrete, having all the qualities of density, tensile 
and crushing strength. 

I am confident that it will be conceded that small stones with 
rounded surfaces can be readily and uniformly mixed, and 
produce a concrete of greater strength and solidity than where 
the stone have irregular faces and sharp edges, as is the case in 
what is known as broken stone. 

Charles EL Haswell, M. Am. Soc. C. E., Ins'n C. E., says : 
" The irregular faces and sharp edges of broken stone are not 
elements of advantage in the composition of either concrete or 
beton, as they in no wise contribute to the adhesion of the lime 
or cement with which they are mixed." 

Making concrete with hydraulic lime or cement will not give 
good results unless there is an affinity with the sand, broken 
stone or pebbles used, which I will illustrate by an extract from 



10 

my report made to the Government of District of Columbia, in 
1886, on the " Sand Test," 

Extract from Report of the Operations of the Engineer De- 
partment of the District of Columbia, for the fiscal year ending 
June 30, 1886 (fol. 57) : 

" Sand Test. 

Hydraulic cement, though tested neat, is hardly, if ever, used 
in that condition, but is mixed with sand to make mortar for the 
construction of walls, etc., or mixed with broken stone, etc., to 
make concrete. As matter of fact a hydraulic cement which 
shows great tensible strength will generally have great power of 
adhesion if fairly employed. 

The desirability of ascertaining the adhesive power of a cement 
has led to what is known as the sand test. It is done by gauging 
cement with sand in certain fixed proportions and testing the 
briquettes, made in the ordinary manner, for tensile strength. 
But it is essential, in order to obtain satisfactory results, that the 
sand should have the required physical and chemical qualities. 

I have seen sands, though clean and sifted through the same 
sieves, and not unlike under the microscope, give results that dif- 
fered greatly when used with the same cements. This led me to 
investigate the causes of the different results apparently obtained 
under similar conditions. 

In Germany the Government sells a standard sand, to all who 
require it, at a moderate price. Here the contractors must obtain 
their sand where and how they can, to be accepted by the Gov- 
ernment under certain specifications. 

Two very essential points in selecting sand for making mor- 
tar should be observed : 

First — That the sand be of a free silicious nature without the 
admixture of carbonaceous sand or clay. Silica and silicates have 
great affinity for hydraulic cements and produce great adhesive 
power, while carbonates, such as carbonates of lime, etc., have 
much less, and give poor results. In a clear washed sand con- 



11 

taining 20% of carbonaceous sand used with hydraulic cement, 
the tensile strength was reduced 30%. After having been treated 
with hydrochloric acid and washed in order to eliminate the car- 
bonates from the sand, its initiative adhesive power was restored. 
Thus in selecting sand a sample ought to be treated with hydro- 
chloric acid, and if any effervescence is perceptible, it is an indi- 
cation that it contains carbonates, and the sand should, therefore, 
be rejected. 

Second — The voids of the sand must be ascertained by first 
drving the sand and then tilling a given measure with it 
well packed and filling the voids with water, the quantity of the 
water used will be equal to the voids in the sand. The quantity 
of the hydraulic cement employed with sand to make mortar 
which will give excellent results should be equal to the volume 
of voids, plus 2%. 

The sand generally employed in the District of Columbia has 
voids from 31 to 32%. The specifications for mortar are two 
parts of sand with one part of cement equals 33|% of hydraulic 
cement. By these proportions the voids are well filled and the 
results obtained are very satisfactory. 

I would therefore recommend that it is important that the 
percentage of cement used in mortar with sand should be at least 
equal to the percentage of voids in the sand. 

A sand having 40% of voids was mixed with 33 \ % of 
hydraulic cement, and the average tensile strength of this (10 
briquettes) was 15.50 pounds per square inch after ten days, 
while the hydraulic cement of the same barrel mixed in the same 
proportions, two of sand to one of cement, the sand having only 
32% of voids, gave in ten days an average tensile strength per 
square inch (10 briquettes) of sixty-two pounds. 

The importance of a sand test cannot be denied, and is, as we 
have seen, indispensable. A great deal has been written on this 
subject without giving much information, but in reality it is a 
simple matter. To test the cement with a standard sand is out 
of the question. The sand to be tested is the sand to be used. 



12 

which, combined with the tests of the hydraulic cements to be 
employed, will give the necessary information as to the ultimate 
strength of the structure. 

Yery respectfully submitted, 

E. J. De Smedt, 
Chemist, District of Columbia, General Inspector 

of Asphalt Pavements and Hydraulic Cement. 

Col. William Ludlow, 

Engineer Commissioner." 

My experiments and observations made since that report was 
submitted, confirms the theory then stated — that hydraulic ce- 
ments have great affinity for silicates and silica, and I can also 
state that what was there reported in reference to the smaller 
grains of silica (sand) apply with equal force to the larger pieces 
of silica — as pebbles or gravel, and that silica stones with rounded 
edges and smooth surfaces will make better concrete or beton 
than broken limestone with sharp edges and of angular forms. 

I wish to be understood that sand or pebbles should be clean 
and thoroughly washed, such as that taken from below the water- 
line on our sea coast. 



ANALYSIS OF GRAVEL 

FROM 

North Shore of Long Island. 

No. 1. 

Silica 94.73% 

Oxide of Iron 1.38% 

Alumina 2.80% 

Lime 0.5i'% 

Magnesia 0.17% 

Phosphorus 0.04% 

Sulphur 0.29% 

ANALYSIS OF GRAVEL 

FROM 

North Shore of Long Island. 
No. 2. 

Silica 80.720% 

Oxide of Iron 5.380% 

Alumina 8.730% 

Lime 1.990% 

Magnesia 1.010% 

Phosphorus 0.074% 

Sulphur 0.260% 

Loss on Ignition 0.980% 

Manganese Trace. 

Undetermined 0.8-26% 



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